The use of air pressure to support the roofs of building structures has been well known for several decades as described, for example, in U.S. Pat. Nos. 2,837,101 and 2,850,026. The so-called "air domes" and various air-supported roofs are known or have been used including roof systems that employ inflatable air bags or bladder means.
Such roof systems have been used in inflatable buildings as in U.S. Pat. No. 2,837,101 and in green-houses and mobile or portable buildings as disclosed in U.S. Pat. Nos. 3,338,000; 4,805,355; 4,924,651 and 4,976,074. Inflatable roofs have been proposed for covering airplane hangers using many tubular air bags mounted side by side as in U.S. Pat. Nos. 4,257,199 and 4,976,074, sometimes using helium in the bags to provide buoyancy as in U.S. Pat. No. 2,850,026.
Also shown in the prior art patents are building structures having inflatable bladder means supported on a fixed ceiling between the ceiling and the roof to support the roof by air pressure as disclosed in U.S. Pat. Nos. 2,837,101; 3,057,368; 4,452,017 and 4,805,355. As pointed out in U.S. Pat. No. 3,057,368 (1962) the air-supported dome can be subject to severe wind and snow loads. For these and other reasons, the use of air domes has been limited.
U.S. Pat. Nos. 4,805,355 and 4,924,651 disclose portable inflatable building structures for use as greenhouses wherein relatively large bladder means of lenticular cross sections are mounted on generally horizontal suspension cables to provide an inflatable bubble roof. Neither patent discloses an inflatable roof suitable for covering a sports arena or other large building.
U.S. Pat. No. 4,976,074 discloses an inflatable roof designed for use as an airplane hanger and described as possibly suitable for covering a sports stadium. However, it appears to be unsafe and impractical for a sports arena.
Sports stadiums have been covered by the so-called "air domes" as disclosed in U.S. Pat. No. 3,744,191, for example, wherein a network of crossing cables is employed to provide the needed strength. Large stadiums have also been covered by "cable domes" having a network of crossing cables in the roof and a similar network of suspension cables with rigid upright struts or spreaders to maintain tension in the cables as disclosed 30 years ago in U.S. Pat. No. 3,410,039.
Spaced radial cables have been proposed for reinforcing the roof as in U.S. Pat. No. 3,740,902 but cable networks have been favored by the experts in stadium technology because of added strength, resistance to vibration and other advantages. The roof systems of U.S. Pat. Nos. 3,410,039; 3,772,836; 3,835,599; 3,841,038; 4,452,017; 4,451,860; 5,259,158;5,371,983 and 5,440,840 employ such cable networks.
While air domes and cable domes, such as those previously mentioned, have been known or proposed for use on sports arenas for decades, the number actually used for that purpose has been limited. A decade ago only seven cities in this country had enclosed domed stadiums for professional sports. However, interest in such stadiums has grown rapidly since the mid-1980s.
The first enclosed stadiums used steel or reinforced concrete domes but such roofs were expensive, heavy and difficult to waterproof. The glazing and trusswork of stadiums, such as the Astrodome, also created problems.
Such problems led to a second generation of air-supported fabric-covered stadium roofs, lightweight and inexpensive structures, capable of covering large spans. Such air domes presented a number of serious problems including high annual costs for heat, electricity and maintenance and excessive crowd noise. The continual positive air pressure necessary to support the roof required that the stadiums have revolving doors, air locks for trucks, reinforced elevator shafts, additional fan rooms, and other expensive and inconvenient items.
Other serious problems stemmed from damage to the roof fabric during construction and deflations of the domed roofs after completion caused by malfunctioning or poorly operated mechanical or snow removal systems. The above-mentioned drawbacks and problems and extensive litigation associated with the air-supported domes discouraged further use of such roof systems for professional sports stadiums so that they are rarely used on new stadiums and are perhaps obsolete.
The demise of air domes led to the third generation of enclosed stadiums that have fabric roofs supported, not by air pressure, but by tension cables held taut either by large masts or arches, or by smaller compression struts and a compression ring. The leading experts in the engineering of these cable domes are David Geiger and Horst Berger (See U.S. Pat. Nos. 3,772,836; 3,807,421; 3,841,038; 4,452,848; 4,581,860; 4,736,553 and 4,757,650).
The advantages of the cable domes over the previous air-supported domes were many. The cable domes are more easily insulated. They also can take greater snow loads and demand less exacting maintenance. Their higher cost remain a major drawback, but the elimination of the equipment and details required for pressurization make them superior to the previous air-supported domes.
The annual costs associated with operation and maintenance of a conventional air dome, such as Pontiac's Silverdome, are enormous and usually substantially greater than one million dollars because of the large volumes of air involved, the difficulty in heating and cooling the structure, and the general inefficiency of the system. These problems are magnified when attempting to design large structures with a clear span of 700 feet or more. In recent years there has been no serious interest in using air-supported roof systems for sports stadiums or large building complexes.
It can be difficult and expensive to provide a durable roof structure with a clear span of 800 feet or more because of the great weight to be supported. The weight of the roof limits the practical maximum size for a dome even when the dome is covered with the conventional Teflon-coated fiberglass fabric, as used on the Georgia dome. Prior to the present invention cable-supported stadium domes with a clear span in excess of 900 feet have not been built because of the enormous expense involved. The experts on stadium technology heretofore have failed to provide a satisfactory solution to the problem or to provide a versatile, safe, reliable roof system that is commercially practical for huge building complexes.